1. Field of the Invention
The present invention relates to a fuel cell including a membrane electrode assembly interposed between separators. The membrane electrode assembly includes a pair of electrodes, and an electrolyte membrane interposed between the electrodes.
2. Description of the Related Art
Generally, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which comprises two electrodes (anode and cathode) and an electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly is interposed between separators. The membrane electrode assembly and the separators make up a unit of the fuel cell for generating electricity. A predetermined number of fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a hydrogen-containing gas is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. An oxygen-containing gas or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
Typically, each of the anode and the cathode has a gas diffusion layer such as a porous carbon paper, and an electrode catalyst layer of platinum alloy supported on porous carbon particles. The carbon particles are deposited uniformly on the surface of the gas diffusion layer. The electrode catalyst layer of the anode and the electrode catalyst layer of the cathode are fixed to both surfaces of the electrolyte membrane, respectively. Japanese patent No. 3,211,378 discloses a polymer electrolyte fuel cell in an attempt to improve the gas diffusion properties of the reactant gases from the gas diffusion layer to the electrode catalyst layer.
In the prior art, a polymer electrolyte membrane is interposed between porous carbon bodies each supporting a catalyst layer. Foamed metals are provided outside the porous carbon bodies, and bulk electrodes are provided outside the foamed metals. Water repellent treatment is applied to at least a part of the foamed metals.
According to the disclosure, diffusion properties of the reactant gases through the foamed metals to the catalyst layers supported by the porous carbon bodies are improved.
Generally, as shown in FIG. 17, the fuel cell of this type has a membrane electrode assembly 1 interposed between a pair of separators 2a, 2b. The membrane electrode assembly 1 includes an anode 4, a cathode 5, and a polymer electrolyte membrane 3 interposed between the anode 4 and the cathode 5. The separator 2a has a reactant gas flow field 6 for supplying a fuel gas to the anode 4. The separator 2b has an oxygen-containing gas flow field 7 for supplying an oxygen-containing gas to the cathode 5.
In the structure, however, the costs of providing the oxygen-containing gas flow field 6 and the fuel gas flow field 7 such as the cost of forming grooves of the separators 2a, 2b, the cost of producing the separators 2a, 2b of metal plates by press forming, and the cost of forming grooves in the diffusion layers of the cathode 4 and the anode 5 are high. Therefore, the overall production cost for the fuel cell is high. Further, the fuel cell has a large dimension in a stacking direction indicated by an arrow X. In particular, the dimension of the fuel cell stack formed by stacking fuel cells in the stacking direction indicated by the arrow X is considerably large.
Typically, reactant gas passages extend through the fuel cell stack as internal manifolds for supplying and discharging reactant gases such as the oxygen-containing gas and the fuel gas. The fuel cell stack requires a sealing structure for reliably preventing the leakage of the reactant gases from the reactant gas passages. The sealing structure tends to be complex, and expensive.
In the prior art, the foamed metal is used as the diffusion layer. The foamed metal has a very low elasticity. Therefore, if the membrane electrode assembly is swelled by absorbing water, or thermally expanded, or if the pressure of impacts or shocks is applied to the surface of the foamed metal, the foamed metal may be plastically deformed undesirably.
If the area in the foamed metal is used as a part of the reactant gas flow field, the porosity of the foamed metal needs to be high for maintaining the pressure loss in the foamed metal. However, if the porosity of the foamed metal is high, the foamed metal is deformed easily due to the low pressure resistance. If the foamed metal has a considerably low resistance, the dimension of the foamed metal changes easily when the load applied to the foamed metal changes during the power generation of the fuel cell. The dimensional change may decrease the surface pressure, and increase the resistance overpotential undesirably.